Dev: Fix issue whereby you couldn't order with a click on a column after using order() API

Fix: I've removed the third entry in the aaSorting array, as the issue
was that after using order() that entry wasn't present. It was a bit
confusing as well, so it is now replaced with a property (_idx on the
aaSorting entries) which indicates the current sorting index (in
asSorting) - and this property is optional. If not given, it is looked
up or 0.